07 May 2020

What are Heatwaves and Where Do They Come From?

Thermometer indicating high temperatures

Heatwaves happen all over the world, although they have different characteristics depending on where you are. Whilst there isn’t a universal definition of exactly what counts as a heatwave, the general consensus is that a heatwave involves at least two consecutive days of unusually hot summer weather. 

Causes of Heatwaves

Heatwaves occur when a system of high atmospheric pressure system moves into an area, pulling air down from the upper levels of the atmosphere and towards the ground. In the process, the air becomes more compressed and its temperature increases. The high pressure also blocks any other weather systems from getting into the same spot, which extends the period of the heatwave.

Winds and clouds are also inhibited by this spot of high-pressure concentration. As a result, there’s nothing to get between the sun’s rays or the earth, or to cool the area by dispersing the heat collecting there. Energy from the sun and radiant heat intensify, and the longer the system stays put, the hotter it gets.

This is why heatwaves last so long – days or even weeks on end. If nothing manages to shift the weather system, the heat keeps intensifying, getting to the point where it’s extreme and very often dangerous to human health.

Why Do High-Pressure Atmospheric Systems Move In?

Of course, for this situation to occur, something has to cause the weather system to move into the area in the first place. A number of factors contribute to this:

1.   The Way the Earth is Tilted

There’s a reason heatwaves tend to happen during the peak summer months. In fact, the reason we have seasons at all is down to the fact that the Earth is tilted on its axis at 23.5°.  
We experience summer when the hemisphere we’re situated on is tilted towards the sun. At this time of year, the sun’s rays directly hit the surface of the Earth, which causes it to heat more intensely than when these rays hit at an angle. What’s more, the days are longer, so there’s not as much time to cool down before you’re back under the glare of the sun. 

While this doesn’t cause heatwaves on its own, it helps to create (or exaggerate) the underlying conditions needed for a heatwave to occur. It also changes the path of jet streams, as we’ll see in a moment.

2.   Movement of the Jet Stream

The Earth is split into two hemispheres, while the majority of energy and heat is focused on the Equator. To spread this heat towards the North and South Poles, air must move about constantly. 

Within the northern and southern hemispheres, this moving air splits up into three cells. Each of these cells circulates air into the lowest section of the atmosphere, called the troposphere. In doing so, warm air is continually moving up and colliding with much colder air near the poles. This creates a slender but fluctuating band of air, flowing to the West and around and around the Earth at high speed and with great force. We call these “jet streams”.

Typically, you get two or three jet streams in each hemisphere. Those in the northern hemisphere are formed between warm air coming up from the south and the cooler air in the north. In the southern hemisphere, they run between warmer air coming down from the north and the colder air in the south. Each jet stream moves over the course of the year.

The jet stream creates winds and changes pressure systems. When it meets an area of low pressure it sucks out air, which increases the pressure. Although these jet streams flow many miles above the Earth’s surface, they have a stark effect on the weather below. 

Over the course of the year, the jet stream shifts. When the temperature difference between the warm and cool air is very large, the jet stream tends to gather speed and force. At these times, when it’s flowing quickly, a jet stream can trigger extreme weather events like dramatic storms.

However, in the summer, there’s isn’t so great a difference between the temperature of the air at the pole of that hemisphere and the Equator. Cold air doesn’t clash with hot air as dramatically, so the jet stream calms down and drifts further up towards the pole. As the jet stream buckles and slows, this can allow high-pressure systems to move into an area. 
As we’ve seen, these high-pressure systems can cause a heatwave. 

3.   The Urban Island Heat Effect

Cities tend to experience much worse heatwaves than rural areas. That’s for three reasons: 

  1. Cities tend to be warmer anyway, thanks to all the heat expended by machines, lighting, computers, people, etc., in a confined space, all around the clock. 
  2. Materials like asphalt and concrete absorb and retain heat.
  3. There’s so little vegetation, which means there’s no water evaporating from the ground that could alter the prevailing weather system.

Health Impacts of a Heatwave

Heatwaves can create very dangerous conditions to live and work in. Without proper protection, you may experience dehydration, heat fatigue, heat cramps, heat exhaustion, heatstroke, and sunburn. At times, the impact can be dire. For example, in 2003, a major heatwave across Europe led to thousands of casualties, as well as economic damage totaling billions of Euros.

Places that regularly experience heatwaves, including large parts of North America, tend to issue warnings and supply emergency resources such as cooling stations and shelters to get out of the heat. However, you will need to take special care when managing teams of people who need to perform intense physical labor, such as construction workers.
One particular challenge is recognizing the dangers and symptoms quickly enough. Often, people feel fine after a day of sun exposure, but the effects accumulate over consecutive days until they fall ill quite suddenly. This leads to thousands of heat-related injuries and illnesses on US construction sites, every year.

Other Effects of a Heatwave

People aren’t the only things on a construction site that struggle in high heat. Certain items of equipment may malfunction or break down entirely. Materials like sealant, glue, and finishes can soften or melt, while concrete and paint may refuse to dry. If there’s high humidity, this can cause damage to wooden components.

Even without this challenge, you may be forced to stop work simply because your materials and equipment can’t cope with the heat. Concrete and paint might not dry. Glue, sealant and other finishing materials may melt or soften. Humidity can damage wood and other materials. Certain units might break down or malfunction in the heat. 

Preparing Your Jobsite for a Heatwave

You can’t always rely on the authorities to issue heatwave warnings, partly because these can be unpredictable and partly because heatwaves are difficult to define. That means you need to take steps to prepare for the risk of a heatwave at any time during the hotter months.

Here are some pointers to get you started:

  • Make it a rule to wear sunscreen on hot, sunny days.
  • Provide helmets and vests that have built-in cooling technology, such as increased airflow, space for ice packs and/or fans.
  • Provide plenty of drinking water. 
  • Schedule regular breaks throughout the day. 
  • Where possible, arrange shifts so that people aren’t working outdoors during the hottest times of the day.
  • Tent off work areas so that workers can get out of direct sunlight. White tents are best as this won’t absorb heat, as dark-colored materials do.
  • Explore dedicated comfort cooling and dehumidification options. as well as tenting off work areas to provide some respite from the sun’s glare.

And Finally…

Plan ahead. Be prepared for the fact that heatwaves frequently cause power outages and equipment failures – and make sure to figure out a comprehensive backup plan to get back up and running if this happens. 

It’s also worth thinking about how the heat may slow down certain tasks or create bottlenecks, pushing your construction project behind schedule. What temporary utilities could you arrange in advance to keep things on track? If you’re unsure, talk to a rental utility company about your concerns and what critical tasks and processes you’ll need to get up and running first in a crisis. Working out how to handle the heat now will save you a lot of stress and hassle when the time comes.